Organic EL elements (also referred to as “organic electroluminescence light emitting elements”), which employ electroluminescence of organic materials (hereinafter referred to as “EL”), have already been put into practice as novel light-emitting systems capable of planar light emission. Organic EL elements have recently been applied to electronic displays and also to lighting apparatuses, and further development of organic EL elements is anticipated.
When an electric field is applied to such an organic EL element, holes and electrons are respectively injected from an anode and a cathode into a light-emitting layer, and the injected holes and electrons are recombined in the light-emitting layer to generate excitons. In this case, singlet excitons and triplet excitons are generated at a ratio of 25%:75%, and thus, phosphorescence, which employs triplet excitons, theoretically provides internal quantum efficiency higher than that of fluorescence. Unfortunately, achievement of high quantum efficiency in a phosphorescent mode requires use of a complex of a rare metal, such as iridium or platinum as a central metal, which may cause future significant problems in the industry in terms of the reserves and price of rare metals.
Meanwhile, various fluorescent elements also have been developed for improving the emission efficiency, and a new movement has occurred in recent years. For example, PTL 1 discloses a technique focused on a triplet-triplet annihilation (TTA) phenomenon (hereinafter also called “triplet-triplet fusion (TTF)”) wherein singlet excitons are generated by collision of two triplet excitons. This technique allows the TTA phenomenon to occur efficiently and thus improves the emission efficiency of a fluorescent element. This technique can increase the emission efficiency of the fluorescent material to two to three times that of a conventional fluorescent material. However, a problem of improving the emission efficiency remains, unlike a phosphorescent material, because singlet excitons are theoretically generated at efficiency of only about 40% by the TTA phenomenon.
In more recent years, fluorescent materials based on a thermally activated delayed fluorescence (hereinafter abbreviated as “TADF” as appropriate) phenomenon, which employs a phenomenon in which reverse intersystem crossing (hereinafter, abbreviated as “RISC” as appropriate) from the triplet excitons to the singlet excitons is caused, and applicability of the materials to organic EL elements has been reported (see, for example, PTL 2 and NPLs 1 and 2). Use of delayed fluorescence by means of this TADF mechanism can theoretically achieve 100% internal quantum efficiency, equivalent to phosphorescence, even in fluorescence caused by electric-field excitation.
In order for development of the TADF phenomenon, it is necessary to cause reverse intersystem crossing from 75% of triplet excitons to singlet excitons generated by electric-field excitation at room temperature or the light-emitting layer temperature in the light-emitting element. Additionally, singlet excitons generated by reverse intersystem crossing emit fluorescence similarly to 25% of singlet excitons generated by direct excitation to enable theoretically 100% internal quantum efficiency to be achieved. In order to cause this reverse intersystem crossing, it is essential that the absolute value of the difference between the lowest singlet excited energy level (S1) and the lowest triplet excited energy level (T1) (hereinafter, referred to as ΔEST) is extremely small.
Meanwhile, it is known that incorporation of a material exhibiting the TADF property as a third component (assist dopant material) into a light-emitting layer including a host material and a light-emitting material is effective for achieving high emission efficiency (see NPL 3). Generation of 25% of singlet excitons and 75% of triplet excitons on the assist dopant by electric-field excitation enables production of singlet excitons through reverse intersystem crossing (RISC). The energy of the singlet excitons is transferred to the luminescent compound by fluorescence resonance energy transfer (hereinafter, abbreviated as FRET, as appropriate), and the luminescent compound can emit light by means of the transferred energy. Thus, use of the theoretically 100% exciton energy enables the luminescent compound to emit light, and high emission efficiency is developed.
Herein, in order to minimize ΔEst in an organic compound, it is known that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are preferably localized in a molecule without mixing the orbitals.
Conventionally, for distinctly separating the HOMO and the LUMO, there is known a technique of introducing a strong electron-donating group (donor unit) or electron-withdrawing group (acceptor unit) into the molecule. Introduction of a strong electron-donating group or electron-withdrawing group, however, generates a strong intramolecular charge-transfer (CT)-type excited state, and is responsible to lengthening of wavelengths (broadening) in an absorption spectrum or emission spectrum of the compound, leading to a problem of difficult control of the emission wavelength. Moreover, in a π-conjugated boron compound having a strong electron-withdrawing group, as the LUMO level is lowered, the HOMO level is also lowered. Thus, when the π-conjugated boron compound is employed in a light-emitting material or the like, there have been a problem in that the host material is difficult to select due to the low HOMO level and LUMO level of the light-emitting material and the balance between carriers during driving of the EL becomes lost.
π-conjugated boron compounds are anticipated as electron-accepting groups for TADF compounds. This is because the π-conjugated boron compounds, which have an electron withdrawing ability weaker than that of commonly used electron-accepting groups such as a cyano group, sulfonyl group, and triazinyl group as well as exhibit excellent electron transportability and a high light-emitting ability, may be utilized in the electron transport layer or light-emitting layer in organic electroluminescent elements.
For example, NPL 4 discloses a TADF compound in which 10H-phenoxaborin is used as an acceptor unit. NPL 5 discloses a TADF compound of a trimesitylborane derivative.
Meanwhile, an issue of such compounds is low stability. Boron, which is a group 13 element, is an electron-deficient element having an empty p-orbital and is thus susceptible to attack by nucleophilic species. Accordingly, boron containing compounds are generally unstable.
There have been reported various methods for improving the thermodynamic stability of π-conjugated boron compounds. For example, in PTL 3 and PTL 5, attempts have been made to employ a compound having a double phenoxaborin skeleton, which is formed by further adding oxygen atoms to the phenoxaborin, in an organic electroluminescent element as means for improving the thermodynamic stability of a π-conjugated boron compound. PTL 4 discloses a compound fully-annulated by carrying out crosslinking in three directions around the boron atom by means of carbon atoms.